Neurological diseases contribute to 40% of the nation's disabled population. Spinal Cord Injury (SCI) affects over 450,000 individuals, with 30 new occurrences each day. 47% of Spinal Cord Injuries cause damage above the C-4 level vertebra of the spinal cord resulting in quadriplegia, like former actor Christopher Reeve. Without the use of the major appendages, patients are typically restricted to assistive movement and communication. Unfortunately, much of the assistive communication technology available to these people is unnatural or requires extensive training to use well.
Amyotrophic Lateral Sclerosis (ALS) afflicts over 30,000 people in the United States. With 5,000 new cases each year, the disease strikes without a clearly associated risk factor and little correlation to genetic inheritance. ALS inhibits the control of voluntary muscle movement by destroying motor neurons located throughout the central and peripheral nervous system. The gradual degeneration of motor neurons renders the patient unable to initiate movement of the primary extremities, including the arms, neck, and vocal cords. Acclaimed astrophysicist, Stephen Hawking has advanced stages of ALS. His monumental theories of the universe would be confined to his mind if he hadn't retained control of one finger, his only means of communicating. However, most patients lose all motor control. Despite these detrimental neuronal effects, the intellectual functionality of memory, thought, and feeling remain intact, but the patient no longer has appropriate means of communication.
Amyotrophic Lateral Sclerosis and Spinal Cord Injury (ALS/SCI) are two of the most prevalent and devastating neurological diseases. Many other diseases have similar detrimental effects: Cerebral Palsy, Aphasia, Multiple Sclerosis, Apraxia, Huntington's Disease, and Traumatic Brain Injury together afflict over 9 million people in the United States. Although the symptoms vary, the life challenges created by these diseases are comparable to SCI/ALS. With loss of motor control, the production of speech can also be disabled in severe cases. Without the use of speech and mobility, neurological disease greatly diminishes quality of life, confining the patient's ideas to his or her own body. Although these individuals lack the capacity to control the airflow needed for audible sound, the use of the vocal cords can remain intact. This creates the opportunity for an interface that can bypass the communicative barriers imposed by the physical disability.
In general, three subsystems are needed to produce audible speech from a constant airflow. First, information from the brain innervates the person's diaphragm, blowing a steady air stream through the lungs. This airflow is then modulated by the opening and closing of the second subsystem, the larynx, through minute muscle movements. The third subsystem includes the mouth, lips, tongue, and nasal cavity through which the modulated airflow resonates.
The process of producing audible speech requires all components, including the diaphragm, lungs and mouth cavity to be fully functional to produce audible speech. However, inaudible speech, which is not mouthed, is also possible using these subsystems. During silent reading, the brain selectively inhibits the full production process of speech, but still sends neurological information to the area of the larynx. Silent reading does not require regulated airflow to generate speech because it does not produce audible sound. However, the second subsystem, the larynx, can remain active.
The muscles involved in speech production can stretch or contract the vocal folds, which changes the pitch of speech and is known as phonation. The larynx receives information from the cerebral cortex of the brain (labeled “1” in FIG. 1) via the Superior Laryngeal Nerve (SLN) (labeled “2” in FIG. 1). The SLN controls distinct motor units of the Cricothyroid Muscle (CT) (labeled “3” in FIG. 1) allowing the muscle to contract or expand. Each motor unit controls approximately 20 muscle fibers which act in unison to produce the muscle movement of the larynx. Other activities involved in speech production include movement of the mouth, jaw and tongue and are controlled in a similar fashion.
The complex modulation of airflow needed to produce speech depends on the contributions of each one of these subsystems. Neurological diseases inhibit the speech production process, as the loss of functionality of a single component can render a patient unable to speak. Typically, an affected patient lacks the muscular force needed to initiate a steady flow of air. Previous technologies attempted to address this issue by emulating the activity of the dysfunctional speech production components, through devices such an electrolarynx or other voice actuator technologies. However, they still require further complex modulation capabilities which many people are no longer capable of. For example, a person both unable to initiate a steady flow of air and lacking proper tongue control would find themselves unable to communicate intelligibly using these other technologies. Despite this communicative barrier, it is possible to utilize the functionality of the remaining speech subsystems in a neural assistive communication technology. This novel technology can be utilized in a number of other useful applications, relevant to people both with and without disabilities.